Moving exhaust below waterline

[jssailem]

Not all issues come from a single source. Explore the various elements and look for solutions that cause the least disruption.

My exhaust tube is low, straight out of the transom (circled in red).

When I bought the boat, powering up to cruise dragged the stern down until the water rose up on the transom. Diesel fumes blew into the stern wave and the cockpit (especially in a following breeze). A line of soot formed on the transom just below the exhaust port. I figured the old prop was over-pitched. The engine was not running efficiently.


I replaced the prop with a CampbellSailor, which improved thrust.

Now the boat moves through the water. The hull stays flat rather than bogging down. No more diesel smell. Exhaust is cleaner. The transom is clean.

You might consider what's causing your situation.

 

[Project_Mayhem]

I forgot to mention that my exhaust port is off to the side. It's a through hull that occasionally dips below the water

Not all issues come from a single source. Explore the various elements and look for solutions that cause the least disruption.

My exhaust tube is low, straight out of the transom (circled in red).
View attachment 240455
When I bought the boat, powering up to cruise dragged the stern down until the water rose up on the transom. Diesel fumes blew into the stern wave and the cockpit (especially in a following breeze). A line of soot formed on the transom just below the exhaust port. I figured the old prop was over-pitched. The engine was not running efficiently.
View attachment 240456 View attachment 240458

I replaced the prop with a CampbellSailor, which improved thrust.
View attachment 240460
Now the boat moves through the water. The hull stays flat rather than bogging down. No more diesel smell. Exhaust is cleaner. The transom is clean.

You might consider what's causing your situation.

I'm pretty sure that my boat is over propped as well. Max RPM is supposed to be 3100 but it's closer to 2500. If memory serves me correctly, it will hit 3100 in neutral

 

[Sail22Capri22]

My Cat42 has the exhaust routed under the waterline close to the transom. I think the design is effective in keeping the exhaust out of the cockpit but maybe I never operated it in conditions causing the exhaust to move into the cockpit.

I can take a photo of it if your interested.

 

[colemj]

That sounds like two anti-syphon loops or am I just reading
this wrong ?

The loop upward from the waterlift and down to the exhaust outlet isn't an antisyphon loop per se because it has no air break in it. It just prevents water from entering when heeled. The engine should have an antisyphon valve in the raw water system that introduces air when shut down.

Both work to keep water out of the engine.

 

[Stu Jackson]

I've noted this before to you about the VAST community of Catalina 30 owners and their Association..

It is inconceivable to me that you can't go to those sources and find the correct prop for your Catalina 30.

Catalina 30 Groups Link
Subscribe at the web forum:
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*All things Catalina 30 and 309! Sponsored by the International Catalina 30 Assn IC30A. FAQs, Q&A, comments, and experiences especially fixes and modifications invited. Share with over 2200 other owners. Website catalina30.com* *Please join the International Catalina 30 Association.

or by sending an email to:
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This comes from this sbo thread by weekendrken:

1992 Catalina 30

Also: www.catalina30.com

I'm pretty sure that my boat is over propped as well. Max RPM is supposed to be 3100 but it's closer to 2500. If memory serves me correctly, it will hit 3100 in neutral

 

[Project_Mayhem]

I've noted this before to you about the VAST community of Catalina 30 owners and their Association..

It is inconceivable to me that you can't go to those sources and find the correct prop for your Catalina 30.

Catalina 30 Groups Link
Subscribe at the web forum:
Catalina30 groups.io Group
*All things Catalina 30 and 309! Sponsored by the International Catalina 30 Assn IC30A. FAQs, Q&A, comments, and experiences especially fixes and modifications invited. Share with over 2200 other owners. Website catalina30.com* *Please join the International Catalina 30 Association.

or by sending an email to:
Catalina30+subscribe@groups.io

This comes from this sbo thread by weekendrken:

1992 Catalina 30

Also: www.catalina30.com

The folding prop is the one that came with the boat. I did not know that the wrong prop could cause issues beyond speed/rpm (thanks @jssailem!). I'm new to diesels